1. Field of the Invention
The present invention generally relates to substrate materials and processes suitable for use in electronic systems. More particularly, this invention relates to a laminate-type ceramic circuit board structure with enhanced mechanical and thermal properties for purposes of improved robustness and thermal management of power circuit devices.
2. Description of the Related Art
A variety of approaches are known for dissipating heat generated by semiconductor devices, such as integrated circuit (IC) chips. One method is to use a flex circuit laminated or bonded to heat sink. If the flex circuit material is sufficiently thin, this approach can employ a highly conductive path formed by plated vias through the flex circuit to the heat sink. Another method is to equip a printed circuit board (PCB) with an innerlayer heat sink that is the same size or are larger than the PCB to provide a large heat sink for the entire board, and rely on conduction through the PCB material to the heat sink beneath. High-power IC chips, such as power flip chips, are often mounted to substrates formed of ceramic materials such as alumina (Al2O3) or another ceramic material that may be modified to promote its heat conduction capability. Ceramic substrates, which conduct and dissipate heat in the vertical direction away from the chip, are limited in their ability to dissipate heat laterally because the thermal conductivities of ceramic materials are relatively low compared to metals and metal-containing materials, though relatively high compared to PCB's.
Laminate-type ceramic substrates known as low temperature co-fired ceramics (LTCC) have a number of process-related advantages that result from their laminate construction. LTCC substrates are conventionally made up of multiple green tapes that are collated (stacked), laminated and fired (co-fired) to form a monolithic ceramic substrate. LTCC substrates have been formed entirely of green tapes that contain glass mixed with a metal powder. For example, U.S. Pat. No. 6,690,583 to Bergstedt et al. discloses a thermally conductive LTCC substrate formed to have surface cavities in which circuit devices are contained. Electrical connections are then made to the devices by depositing a dielectric layer over the LTCC substrate and the devices within its cavities, and then forming contacts through the dielectric layer to the devices. In this manner, heat is conducted away from the devices through the thermally-conductive LTCC on which they are mounted.
In other applications where individual layers of an LTCC substrate are to carry conductor patterns, resistors, etc., ceramic layers are formed by firing green tapes containing only a mixture of glass and ceramic fillers in a binder. Thick-film conductors, resistors, etc., are printed on individual tapes prior to collating and laminating the tapes. The tapes, along with their conductors and resistors, are then co-fired, during which organic binders within the laminate stack are burned off and the remaining materials form, according to their compositions, ceramic and metallic materials. Though LTCC substrates of this type have a number of process-related advantages resulting from their laminate construction, they typically have about half the mechanical strength of a comparable alumina substrate and thermal conductivities of about 3 W/mK, compared to about 24 W/mK for alumina. Mechanical strength can be improved by providing “dummy” glass dielectric layers within the laminate stack, resulting in a thicker, more robust LTCC substrate. Improvements in thermal conductivity have been obtained with the use of thermal vias. As represented in FIG. 1, this approach involves forming multiple vias 114 through the thickness of an LTCC substrate 110 to conduct heat in a vertical direction from a power chip 112. The thermal vias 114 are formed by punching vias in each green tape and then filling the vias prior to printing the conductors, resistors, etc. Interconnect vias 116 required for the LTCC substrate 110 can be formed and filled at the same time as the thermal vias 114. The tapes are then laminated so that the filled vias are aligned to form through-vias, after which the tapes are fired such that the via fill material is co-fired along with conductor and resistor materials printed on surfaces of individual tapes. The entire LTCC substrate 110 is then bonded with an adhesive 118 to a heat sink 120 so that the thermal vias 114 conduct heat from the chip 112 to the heat sink 120.
While able to promote the conduction of heat away from power devices, thermal vias incur additional processing and material costs, reduce routing density, and can limit design flexibility. Furthermore, the thermal vias may be inadequate to achieve suitable thermal management of the power device. Alternatives to conventional filled thermal vias have been proposed. For example, U.S. Pat. No. 5,386,339 to Polinski, Sr., discloses making a thermally conductive path through an otherwise conventional LTCC by defining a hole in the LTCC that is filled with a number of LTCC green tapes containing a thermally conductive material, such as a metal powder dispersed in the mixture of glass and ceramic fillers. On firing, the tapes form an LTCC substrate in which a vertical thermally-conductive path is formed by the tapes containing the conductive material.
Notwithstanding the above, further improvements in the construction and processing of LTCC substrates would be desirable to improve thermal management of power IC's while retaining the process-related advantages of LTCC's.